Semiconductor memory and method of manufacturing the same

ABSTRACT

A semiconductor memory includes a plurality of stripe-like active areas formed by stacking, in a direction perpendicular to a substrate, a plurality of layers extending parallel to the substrate, a first gate electrode formed on first side surfaces of the active areas, the first side surfaces being perpendicular to the substrate, a second gate electrode formed on second side surfaces of the active areas, the second side surfaces being perpendicular to the substrate. The layers are patterned in self-alignment with each other, intersections of the active areas and the first gate electrode form a plurality of memory cells, and the plurality of memory cells in an intersecting plane share the first gate electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of Application Ser. No. 11/858,731, filed Sep.20, 2007, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory and a method ofmanufacturing the same.

2. Description of the Related Art

Flash memories are widely used to store large-volume data in, e.g., cellphones, digital still cameras (DSCs), USB memories, and silicon audio,and the markets of these flash memories keep extending due to thereduction in manufacturing cost per bit (bit cost) resulting from rapidscaling of the device dimension. New applications are also rapidlyrising. The result is a favorable cycle in which the rapid scaling-downand the reduction in manufacturing cost find new markets.

In particular, a NAND flash memory has achieved a practical cross-pointcell by allowing a plurality of active areas (AAs) to share a gateelectrode (GC), and its simple structure allows rapid progress ofscaling. NAND flash memories are beginning to be widely used for storagepurposes in, e.g., the USB memories and silicon audio described above,since the above-mentioned rapid scaling-down reduces the bit cost.Accordingly, the recent NAND flash memories are leading devices of LSI(Large Scale Integration) scaling, and the minimum half pitch hasreached 0.1 μm or less even on the mass-production level. Although thetechnical difficulties are also abruptly increasing with the rapidscaling of the dimension, demands are arising for further scaling in thefuture.

Unfortunately, many problems must be solved to further scaling of flashmemories. The problems are enumerated below:

(1) The development of lithography techniques cannot follow the rapiddevice scaling. Presently, mass-production of lithography apparatusesstarts immediately after they are put on sale. In the future, therefore,it is necessary to increase the bit density while keeping thelithography techniques in status quo.

(2) Since the dimensions of elements decrease as micropatterningprogresses, the short-channel effect or narrow-channel effect abruptlyworsens. This makes it difficult to ensure the reliability and increasethe operating speed of nonvolatile memories generation by generation.

(3) As scaling advances, the dimensions of elements decrease. Therefore,statistical variations in numbers of atoms of dopant impurities oftransistors and the like presumably worsen the device characteristics orthe variations in device characteristics in the future.

Accordingly, it is highly likely to become difficult to continuouslyincrease the bit density in the future by simple scaling of elementssize in the horizontal plane only.

The present inventor, therefore, has invented a stacked memory as asemiconductor memory structure capable of relatively easily increasingthe bit density of memory elements, without entirely depending uponmicropatterning of the lithography techniques, and a method ofmanufacturing the stacked memory.

As well-known examples of stacked memories, methods of sequentiallystacking memory layers as described in patent references 1 to 8, andsome stacked memories are presently mass-produced. However, any of thesemethods forms memory layers by stacking one layer at a time. If thenumber of memory layers increases, therefore, the number ofmanufacturing steps largely increases.

[Patent reference 1] Jpn. Pat. Appln. KOKAI Publication No. 7-235649

[Patent reference 2] U.S. Pat. No. 6,534,403B2

[Patent reference 3] United States Patent Application Publication Pub.No. US2005/0014334A1

[Patent reference 4] United States Patent Application Publication Pub.No. US2005/0012119A1

[Patent reference 5] United States Patent Application Publication Pub.No. US2005/0012120A1

[Patent reference 6] United States Patent Application Publication Pub.No. US2005/0012154A1

[Patent reference 7] United States Patent Application Publication Pub.No. US2005/0012220A1

[Patent reference 8] United States Patent Application Publication Pub.No. US2005/0014322A1

BRIEF SUMMARY OF THE INVENTION

A semiconductor memory according to the first aspect of the presentinvention comprises a plurality of stripe-like active areas formed bystacking, in a direction perpendicular to a substrate, a plurality oflayers extending parallel to the substrate, a first gate electrodeformed on first side surfaces of the active areas, the first sidesurfaces being perpendicular to the substrate, a second gate electrodeformed on second side surfaces of the active areas, the second sidesurfaces being perpendicular to the substrate, and wherein the layersare patterned in self-alignment with each other, intersections of theactive areas and the first gate electrode form a plurality of memorycells, and the plurality of memory cells in an intersecting plane sharethe first gate electrode.

A semiconductor memory manufacturing method according to the secondaspect of the present invention comprises depositing a plurality oflayers on a substrate, forming a plurality of stripe-like active areasby processing the layers in self-alignment with each other, and forminga plurality of gate electrodes intersecting the active areas in alongitudinal direction thereof, wherein each of the active areas uses,as a channel region, at least one of two side surfaces perpendicular tothe substrate, intersections of the active areas and the gate electrodesform memory cells, and a plurality of memory cells in an intersectingplane share the gate electrode.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a perspective view showing a main manufacturing step of asemiconductor memory according to the first embodiment of the presentinvention;

FIG. 2 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the first embodiment of the presentinvention;

FIG. 3 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the first embodiment of the presentinvention;

FIG. 4 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the first embodiment of the presentinvention;

FIG. 5 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the first embodiment of the presentinvention;

FIG. 6 is a plan view showing a main manufacturing step of thesemiconductor memory according to the first embodiment of the presentinvention;

FIG. 7 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the first embodiment of the presentinvention;

FIG. 8 is a sectional view showing a main manufacturing step of asemiconductor memory according to the second embodiment of the presentinvention;

FIG. 9 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the second embodiment of the presentinvention;

FIG. 10 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the second embodiment of the presentinvention;

FIG. 11 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the second embodiment of the presentinvention;

FIG. 12 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the second embodiment of the presentinvention;

FIG. 13 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the second embodiment of the presentinvention;

FIG. 14 is a sectional view showing a main manufacturing step of asemiconductor memory according to the third embodiment of the presentinvention;

FIG. 15 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the third embodiment of the presentinvention;

FIG. 16 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the third embodiment of the presentinvention;

FIG. 17 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the third embodiment of the presentinvention;

FIG. 18 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the third embodiment of the presentinvention;

FIG. 19 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the third embodiment of the presentinvention;

FIG. 20 is a perspective view showing a main manufacturing step of asemiconductor memory according to the fourth embodiment of the presentinvention;

FIG. 21 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the fourth embodiment of the presentinvention;

FIG. 22 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the fourth embodiment of the presentinvention;

FIG. 23 is a perspective view showing a main manufacturing step of asemiconductor memory according to the fifth embodiment of the presentinvention;

FIG. 24 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the fifth embodiment of the presentinvention;

FIG. 25 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the fifth embodiment of the presentinvention;

FIG. 26 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the fifth embodiment of the presentinvention;

FIG. 27 is a sectional view showing a plane perpendicular to AA regionsof the semiconductor memories according to the first to thirdembodiments;

FIG. 28 is a sectional view showing a plane perpendicular to AA regionsof the semiconductor memory according to the fourth embodiment;

FIG. 29 is a sectional view showing a plane perpendicular to AA regionsof the semiconductor memory according to the fifth embodiment;

FIG. 30 is a sectional view showing a main manufacturing step of asemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 31 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 32 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 33 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 34 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 35 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 36 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 37 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 38 is a perspective view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 39 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 40 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the sixth embodiment of the presentinvention;

FIG. 41 is a sectional view of a semiconductor memory according to amodification of the sixth embodiment of the present invention;

FIG. 42 is a sectional view showing a main manufacturing step of asemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 43 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 44 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 45 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 46 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 47 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 48 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 49 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 50 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 51 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 52 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the seventh embodiment of the presentinvention;

FIG. 53 is a sectional view showing a main manufacturing step of asemiconductor memory according to the eighth embodiment of the presentinvention;

FIG. 54 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the eighth embodiment of the presentinvention;

FIG. 55 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the eighth embodiment of the presentinvention;

FIG. 56 is a sectional view showing a main manufacturing step of asemiconductor memory according to the ninth embodiment of the presentinvention;

FIG. 57 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the ninth embodiment of the presentinvention;

FIG. 58 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the ninth embodiment of the presentinvention;

FIG. 59 is a sectional view showing a semiconductor memory according toa modification of the ninth embodiment of the present invention;

FIG. 60 is a sectional view showing a main manufacturing step of asemiconductor memory according to the 10th embodiment of the presentinvention;

FIG. 61 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the 10th embodiment of the presentinvention;

FIG. 62 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the 10th embodiment of the presentinvention;

FIG. 63 is a sectional view showing a main manufacturing step of asemiconductor memory according to the 11th embodiment of the presentinvention;

FIG. 64 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the 11th embodiment of the presentinvention; and

FIG. 65 is a sectional view showing a main manufacturing step of thesemiconductor memory according to the 11th embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Outlines of semiconductor memories and methods of manufacturing the sameto be disclosed in the following embodiments are:

A semiconductor memory that is a stacked memory in which a plurality ofactive area (AA) layers are stacked parallel to a substrate, and thesestacked AAs are controlled by a common gate electrode (GC).

A semiconductor memory manufacturing method comprising a step ofstacking, as AA layers, a plurality of single-crystal silicon layers,polysilicon layers, or amorphous silicon layers to be finally convertedinto polysilicon layers, via interlayer dielectrics or single-crystalsilicon germanium films, and simultaneously processing the stackedlayers, thereby forming a plurality of AAs stacked parallel to asubstrate, and a step of forming a conductor film serving as a GC on theentire substrate surface, and processing the conductor film by reactiveion etching (RIE) or chemical-mechanical polishing (CMP), therebyforming GCs of the stacked AAs at once.

The stacked memory and the method of manufacturing the same describedabove can further increase the bit density compared to the conventionalmemories and methods.

Also, the stacked memory has layer selection gate transistors forselecting a layer among the stacked layers. This makes it possible touse peripheral circuits without largely changing the conventional flashmemories, while the number of times of lithography is small and the bitdensity is high.

Several embodiments of the present invention will be explained belowwith reference to the accompanying drawing. Note that the same referencenumerals denote the same parts in the drawing.

(First Embodiment)

Each of FIGS. 1 to 7 is a perspective, sectional, or plan view showing asemiconductor memory (flash memory) according to the first embodiment ofthe present invention in a predetermined main manufacturing step.

This embodiment is directed to a two-layered memory in whichsingle-crystal silicon layers serving as AA regions are formed bysolid-phase epitaxial growth.

First, as shown in FIG. 1, a silicon thermal oxide film 102 serving asan interlayer dielectric (ILD) is formed to have a thickness of, e.g.,about 50 nm on a semiconductor substrate 101. Then, the conventionallithography technique and etching technique are used to expose portionsof the substrate as seeds of solid-phase growth. An amorphous siliconfilm about, e.g., 60 nm thick is formed on the entire surface of thesubstrate. Subsequently, annealing is performed to cause solid-phaseepitaxial growth of the amorphous silicon film from the exposed portionsof the substrate, thereby forming a single-crystal silicon film 103about, e.g., 60 nm thick on the thermal oxide film 102. A CVD siliconoxide film 104 serving as an interlayer dielectric is formed to have athickness of, e.g., about 50 nm. Then, the conventional lithographytechnique and RIE technique are used to expose portions of the substrateas seeds of solid-phase growth again. An amorphous silicon film about,e.g., 60 nm thick is formed on the entire surface of the substrate.Subsequently, annealing is performed to cause solid-phase growth of theamorphous silicon film from the exposed portions of the substrate,thereby forming a single-crystal silicon film 105 about, e.g., 60 nmthick on the CVD silicon oxide film 104. After that, a silicon thermaloxide film 106 about, e.g., 2 nm thick is formed on the single-crystalsilicon film 105, a silicon nitride film 107 about, e.g., 60 nm thick isformed on the silicon thermal oxide film 106, and a CVD silicon oxidefilm 108 about, e.g., 100 nm thick is formed on the silicon nitride film107.

Then, as shown in FIG. 2, the conventional lithography technique and RIEtechnique are used to sequentially remove the CVD silicon oxide film108, silicon nitride film 107, silicon thermal oxide film 106,single-crystal silicon film 105, CVD silicon oxide film 104,single-crystal silicon film 103, and silicon thermal oxide film 102 froma peripheral circuit portion, thereby exposing the surface of thesemiconductor substrate 101. A thermal oxide film 109 serving as a gateoxide film of a high-voltage transistor is formed to have a thicknessof, e.g., about 35 nm on the exposed surface of the semiconductorsubstrate 101. Subsequently, the conventional lithography technique andwet etching are used to remove the thermal oxide film 109 from alow-voltage transistor region. On this region from which the thermaloxide film 109 is removed, a thermal oxide film 110 serving as a gateoxide film of a low-voltage transistor is formed to have a thicknesssmaller than that of the thermal oxide film 109. After that, apolysilicon film 111 serving as a gate electrode is formed to have athickness of, e.g., 100 nm on the entire substrate surface.

As shown in FIG. 3, the conventional transistor formation procedure isused to form transistors 112 and shallow trench isolations (STIs) 113 inthe peripheral circuit portion. An interlayer dielectric 114 is formedon the entire substrate surface, and the upper surface of the interlayerdielectric 114 is planarized.

Subsequently, as shown in FIG. 4, the conventional lithography techniqueand RIE technique are used to process the polysilicon film 111 and CVDsilicon oxide film 108. The processed polysilicon films 111 and CVDsilicon oxide films 108 are used as hard masks to sequentially processthe silicon nitride film 107, CVD silicon oxide film 106, single-crystalsilicon film 105, CVD silicon oxide film 104, and single-crystal siliconfilm 103, and over-etch the silicon thermal oxide film 102, therebyforming stripe patterns serving as AA regions of a multilayered memory.

As shown in FIG. 5, the end faces of the AA regions exposed by theprocessing are thermally oxidized, and the thermal oxide films areremoved by dry pre-treatment, thereby removing the processing damage.Then, silicon thermal oxide films/CVD silicon nitride films/ALD-aluminafilms 115 serving as memory dielectric layers are sequentially formed.Subsequently, phosphorus (P)-doped polysilicon films 116 serving as gateelectrodes are filled between the AAs. This embodiment processes thegate electrodes by using the conventional lithography technique andreactive ion etching. Consequently, the stacked AA regions share thegate electrodes.

As shown in FIG. 6, diffusion layers serving as source/drain regions areformed by gas phase doping (GPD), and spin-on-glass (SOG) films 117serving as interlayer dielectrics are filled between the gate electrodesmade of the polysilicon films 116. Then, contact plugs connecting to theAA regions and gate electrodes (GCs) are formed. That is, theconventional lithography technique and RIE technique are used topartially remove the upper AA regions of the stacked AA regions, and theinterlayer dielectrics 117 are filled. After the upper surfaces of theinterlayer dielectrics 117 are planarized, contact plugs 118 and 119 areformed. The contact plugs 118 correspond to the upper AA regions (AA21to AA25), and the contact plugs 119 correspond to the lower AA regions(AA11 to AA15). One contact plug is formed over two AA regions, and thecontact plugs at the two ends of each AA region are in zigzag alignmentwith each other by one active area. Accordingly, an arbitrary AA regioncan be selected by selecting two contact plugs. FIG. 6 is a plan viewshowing the arrangement of the contact plugs.

Multilayered interconnection formation steps start after the formationof the contact plugs. These steps form interlayer dielectrics 120, 123,and 126, interconnections 122, 125, and 127, and contact plugs 121 and124. Although a flash memory is formed by repeating the above procedure,the details will not be explained. FIG. 7 shows the final shape.

This embodiment achieves the storage density twice that of theconventional memories. Although the AA regions are stacked on thesubstrate, each of AA region processing and GC processing need only beperformed once as in the conventional unstacked memories.

In this embodiment, the source/drain area of the cell transistor isformed by GPD. The cell transistor is SOI structure. Because of this, itis possible that it is used as a cell transistor of depletion type. Forexample, instead of forming the source/drain area by doping, it dopesimpurities such as P at the time of forming an amorphous silicon film orit dopes impurities such as P after forming the single-crystal siliconfilm 103. By this process, impurities are doped in the single crystalsilicon film 103 of the cell region uniformly, the depletion layer whichappears by providing a voltage in the gate electrode is used, and ON/offof the cell transistor works.

Also, in the structure of the present invention, each cell transistorhas an SOI structure, and this SOI structure is a double-gate structurein which the gate electrodes (GCs) sandwich the AA region. This makesthe structure of the present invention tough against the short-channeleffect.

Furthermore, the channel region is formed perpendicularly to thesubstrate, and hence the channel width can be set regardless of thedesign rules. This achieves the advantage that it is possible tomanufacture a transistor tough against the narrow-channel effect aswell.

As described above, the structure of this embodiment can increase thebit density without any micropatterning, and does not increase thenumber of critical lithography steps which are essential for minimumhalf pitch patterning.

(Second Embodiment)

Each of FIGS. 8 to 13 is a perspective or sectional view showing asemiconductor memory (flash memory) according to the second embodimentof the present invention in a predetermined main manufacturing step.

This embodiment is directed to a four-layered memory in whichsingle-crystal silicon layers serving as AA regions are formed bysequentially stacking and growing epitaxial silicon and epitaxialsilicon germanium, and removing the silicon germanium films by selectiveetching.

First, as shown in FIG. 8, a first epitaxial silicon germanium film 202about, e.g., 50 nm thick, first epitaxial silicon film 203 about, e.g.,60 nm thick, second epitaxial silicon germanium film 204 about, e.g., 50nm thick, second epitaxial silicon film 205 about, e.g., 60 nm thick,third epitaxial silicon germanium film 206 about, e.g., 50 nm thick,third epitaxial silicon film 207 about, e.g., 60 nm thick, fourthepitaxial silicon germanium film 208 about, e.g., 50 nm thick, andfourth epitaxial silicon film 209 about, e.g., 70 nm thick aresequentially formed on a semiconductor substrate 201. Then, a plasma CVDsilicon oxide film about, e.g., 100 nm thick is formed on the entiresubstrate surface. The conventional lithography technique and reactiveion etching are used to simultaneously process the plasma CVD siliconoxide film and layered epitaxial films in a cell region. Subsequently,an epitaxial silicon film 210 about, e.g., 100 nm thick is formed on theexposed end faces of the layered epitaxial films. The conventionallithography technique and RIE technique are used to remove the plasmaCVD silicon oxide film from a contact plug region. The remaining plasmaCVD silicon oxide film is used as a mask to etch the epitaxial silicongermanium films and epitaxial silicon films with an aqueous alkalinesolution. Since the etching rate changes from one crystal orientation toanother, the stacked epitaxial films are facet-etched at an inclinationangle of 45° as shown in FIG. 8. On the exposed surface of thesemiconductor substrate 201, a thermal oxide film 211 serving as a gateoxide film of a high-voltage transistor is formed to have a thicknessof, e.g., about 35 nm. Subsequently, the conventional lithographytechnique and wet etching are used to remove the thermal oxide film 211and the above-mentioned plasma CVD silicon oxide film from a low-voltagetransistor region, and a thermal oxide film 212 serving as a gate oxidefilm of a low-voltage transistor is formed. A 100-nm thick polysiliconfilm serving as a gate electrode is formed on the entire substratesurface. Then, as in the first embodiment, the conventional transistorformation procedure is used to process the polysilicon film, formdiffusion layers serving as source/drain regions, form transistors 213and STIs 214 in a peripheral circuit portion, and selectively form asilicon nitride film 215 in the cell portion. After that, an interlayerdielectric 216 is formed on the entire substrate surface, and the uppersurface of the interlayer dielectric 216 is planarized.

Then, as shown in FIG. 9, the conventional lithography technique andreactive ion etching are used to simultaneously process the interlayerdielectric film 216, silicon nitride film 215, and layered epitaxialfilms in the cell region into stripe patterns of AA regions. Unlike inthe first embodiment, the epitaxial silicon films serving as AA regionsare stacked via the epitaxial silicon germanium films. Since this makesRIE processing relatively easy, it is possible to form even the stripepatterns of AA regions of a multilayered memory having a large number oflayers.

As shown in FIG. 10, only the epitaxial silicon germanium films of thelayered epitaxial films are removed by selective etching. The selectiveetching is wet etching by mixture of fluoric acid and nitric acid or gasetching with CF₄/O₂ system. This forms a shape in which the linear AAregions float as they are supported at their two ends by the epitaxialsilicon film 210 and the silicon nitride film 215 formed in thefacet-etched region. Then, an SOG film 217 is formed by coating andchanged into an oxide film under appropriate annealing conditions,thereby filling all the spaces between the linear AA regions with theSOG film 217. Although this embodiment employs the SOG film filled inthe inter-layer spaces, it is also possible to fill a flowabledielectric by Chemical Vapor Condensation or the like. Also, theepitaxial silicon film 203 about, e.g., 60 nm thick, the epitaxialsilicon germanium film 204 about, e.g., 20 nm thick are formed. Theepitaxial silicon germanium film 204 is removed by etching to form thespace. The epitaxial silicon film 203 on the top and bottom of the spaceis thermally oxidized to form the thermally oxide film between theepitaxial silicon films 203. Although this embodiment can be filled theinsulating film in the inter-layer spaces.

Subsequently, as shown in FIG. 11, the SOG film 217 is etched back byreactive ion etching and left behind only between the vertically stackedAA regions. This forms interlayer dielectrics between the stacked AAregions.

As shown in FIG. 12, the end faces of the AA regions exposed by theprocessing are thermally oxidized, and the thermal oxide films areremoved by dry pre-treatment, thereby removing the process damage. Then,thermal oxide films/CVD silicon nitride films/ALD-hafnium oxide films218 serving as memory dielectric layers are sequentially formed.Subsequently, phosphorus (P)-doped polysilicon films 219 serving as gateelectrodes are filled between the AA regions. This embodiment processesthe gate electrodes by using the conventional lithography technique andreactive ion etching. This forms gate electrodes (GCs) that the stackedAA regions share in the plane perpendicular to the AA regions.

As shown in FIG. 13, diffusion layers are formed by gas phase doping,and an SOG film 220 serving as an interlayer dielectric is filledbetween the gate electrodes and planarized. Next, silicidation of GCs isperformed. In case of conventional stacked memory which is manufacturedby sequential stacking of individual memory layer, underlying memorylayer is influenced with the thermal budget of upper memory layerformation, therefore it is difficult to employ GC silicidation processwhich is fragile against thermal treatment. This invention has anadvantage of easier employment of silicidation since silicidation shouldbe done only at once after GCs formation. After that, contact plugs 221connecting to the AA regions and CG electrodes are formed. Inparticular, contact plugs connecting to the AAs can be simultaneouslyformed because the end portions of the AA regions are offset by facetetching. After the contact plugs are formed, multilayeredinterconnection formation steps begin. These steps form interlayerdielectrics 222 and 225, interconnections 223 and 226, and contact plugs224, thereby forming a flash memory. However, the details will not beexplained, and only the final shape is shown. Note that the SOG film 220is intentionally unshown in the cell portion illustrated in FIG. 13 inorder to clearly show the relationship between the AA regions and gateelectrodes (GCs).

This embodiment achieves the storage density four times that of theconventional memory having only one memory layer. Although the AAregions are stacked on the substrate, each of AA region processing andGC processing need only be performed once as in the conventional memoryhaving only one storage layer. The result is the advantage that thenumber of processing steps does not largely increase.

Also, similar to the first embodiment, the structure of the presentinvention is tough against the short-channel effect and narrow-channeleffect.

In this embodiment, similar to the first embodiment, it is possible thatit is used as a cell transistor of depletion type. For example, insteadof forming the source/drain area by doping, it dopes impurities such asP at the time of forming the epitaxial silicon films 203 or it dopesimpurities such as P after forming the epitaxial silicon films 203. Bythis process, impurities are doped in the epitaxial silicon films 203 ofthe cell region uniformly, the depletion layer which appears byproviding a voltage in the gate electrode is used, and ON/off of thecell transistor works.

Also, the epitaxial silicon germanium film is removed by selectiveetching after the lamination layer of the epitaxial silicon film/theepitaxial silicon germanium film process to cell size bymicropatternnig. Accordingly the etching amount is little. With aviewpoint of processing of RIE, the minute processing whose precision ishigh is possible because of the lamination layer of silicon/silicongermanium which is the same kind of film. In stead of this, beforeprocessing it in full detail to the cell size, an active area isprocessed to the size of (about the cell area) of about a severalmicrons×several microns. The epitaxial silicon germanium film is removedfrom the processed end part by wet etching under the condition. Afterthe insulating (misspelling) film is formed in that gap, it is possiblethat it is processed into the line-shaped in full detail as well as thefirst embodiment.

As described above, the structure of this embodiment can increase thebit density without any special micropatterning, and does not increasethe number of lithography steps (generally, AA processing, GCprocessing, formation of contact plugs to cells, and extraction ofinterconnections from the contact plugs in a memory) requiring theminimum half pitch, particularly, the numbers of AA processing steps andGC processing steps.

(Third Embodiment)

Each of FIGS. 14 to 19 is a perspective or sectional view showing asemiconductor memory (flash memory) according to the third embodiment ofthe present invention in a predetermined main manufacturing step.

This embodiment is directed to a four-layered memory similar to thesecond embodiment, but gate electrodes are processed by using the CMPtechnique.

First, as shown in FIG. 14, following the same procedure as in thesecond embodiment, four epitaxial silicon germanium films 302 and fourepitaxial silicon films 303 are alternately stacked on a semiconductorsubstrate 301, and a silicon nitride film 304 and plasma CVD siliconoxide film 305 are formed on the layered epitaxial films. Then, theconventional lithography technique and etching technique are used toexpose the end portions of the layered epitaxial films in a cellportion, and an epitaxial silicon film 306 is formed on the exposedportions. The silicon nitride film 304 and plasma CVD silicon oxide film305 are removed from a contact plug region by using the conventionallithography technique and RIE technique. As a consequence, a recess isformed in the contact plug region. Subsequently, an SOG film 307 isformed by coating so as to have a thickness with which the SOG film 307does not fill the recess, thereby forming a gentle slope shape as shownin FIG. 14.

Then, as shown in FIG. 15, the SOG film 307 is used as a mask to etchback the stacked epitaxial films. Since the mask film thickness changesfrom one portion to another, the stacked epitaxial films are processedinto a shape reflecting the gentle slope shape of the SOG film 307 asshown in FIG. 15. Following the same procedure as explained in thesecond embodiment, transistors 308 and STIs 309 of high- and low-voltagecircuits are formed, an interlayer dielectric 310 is formed, and theupper surface of the interlayer dielectric 310 is planarized.

As shown in FIG. 16, similar to the second embodiment, the conventionallithography technique and reactive ion etching are used tosimultaneously process the interlayer dielectric film 310, siliconnitride film 304, and layered epitaxial films in the cell region intostripe patterns of AA regions.

Subsequently, as shown in FIG. 17, only the epitaxial silicon germaniumfilms 303 of the layered epitaxial films are removed by selectiveetching. The selective etching is wet etching by mixture of flioroc acidand nitric acid or gas etching with CF₄/O₂ system. This forms a shape inwhich the linear AA regions float as they are supported at their twoends by the epitaxial silicon film 306 and interlayer dielectric 310.Then, SOG is formed by coating and changed into an oxide film underappropriate annealing conditions, thereby filling all the spaces betweenthe linear AA regions with an SOG film 311. Since an impurity is dopedinto the SOG film 311, diffusion layers are formed by contact diffusionof this impurity. The conventional lithography technique and reactiveion etching are used to etch back the interlayer dielectric 310 and SOGfilm 311, thereby forming gaps as templates of gate electrodes.Consequently, interlayer dielectrics are formed between the verticallystacked AA regions and between adjacent gate electrodes (GC).

As shown in FIG. 18, the end faces of the AA regions exposed by theprocessing are thermally oxidized, and the thermal oxide films areremoved by dry pre-treatment, thereby removing the process damage. Then,thermal oxide films/CVD silicon nitride films/ALD-lanthanum aluminumoxide films 312 serving as memory dielectric layers are sequentiallyformed. Subsequently, CVD titanium nitride films 313 serving as gateelectrodes are filled in the gaps, and planarized by CMP. Since thisembodiment processes the gate electrodes by CMP, metal electrodes can beeasily processed. In addition, the use of metal electrodes is relativelyeasy because the damage of GC processing requiring a relatively hightemperature can be removed before the metal electrodes are filled.

As shown in FIG. 19, an interlayer dielectric 314 is formed andplanarized, and contact plugs 315 connecting to the AA regions and gateelectrodes (GCs) are formed. Contact plugs connecting to the AA regionscan be simultaneously formed because the end portions of the AA regionsare offset into the shape of a gentle slope by etching using the SOGfilm. After the contact plugs are formed, a multilayered interconnectionis formed by forming interlayer dielectrics 316 and 319,interconnections 317 and 320, and contact plugs 318, thereby forming aflash memory. However, the details will not be explained, and only thefinal shape is shown.

Similar to the second embodiment, this embodiment achieves the storagedensity four times that of the conventional memory having only onememory layer. Although this embodiment forms the AA regions by stackingfour layers on the substrate, each of AA processing and GC processingneed only be performed once as in the conventional memory having onlyone memory layer.

Also, similar to the first embodiment, the structure of this embodimentis tough against the short-channel effect and narrow-channel effect.

As described above, this embodiment can increase the degree ofintegration of cells without any micropatterning, and does not increasethe number of lithography steps requiring the minimum half pitch.

(Fourth Embodiment)

FIGS. 20 to 22 are perspective views showing a semiconductor memory(flash memory) according to the fourth embodiment of the presentinvention in predetermined main manufacturing steps.

This embodiment is directed to a four-layered memory similar to thethird embodiment, but implements a dual-gate electrode structure.

First, as shown in FIG. 20, layered epitaxial films are formed byalternately stacking four epitaxial silicon germanium films and fourepitaxial silicon films 402 on a semiconductor substrate 401 followingthe same procedure as in the third embodiment, and a peripheral circuitportion is formed in the same manner as in the first to thirdembodiments. Subsequently, following the same procedure as in the thirdembodiment, a silicon nitride film 403 and the layered epitaxial filmsin a cell region are simultaneously processed into stripe patterns of AAregions by using the conventional lithography technique and reactive ionetching, and only the epitaxial silicon germanium films of the layeredepitaxial films are removed by selective wet etching. Then, SOG isformed by coating and changed into an oxide film under appropriateannealing conditions, thereby filling all the spaces between the linearAA regions with SOG films 404. The conventional lithography techniqueand reactive ion etching are used to etch back the silicon oxide filmdescribed above, thereby forming gaps as templates of gate electrodes.Unlike in the previous embodiments, the width of the template is “3F”(“F” is the minimum half pitch) in this embodiment.

As shown in FIG. 21, the end faces of the AA regions exposed by theprocessing are thermally oxidized, and the thermal oxide films areremoved by dry pre-treatment, thereby removing the process damage. Then,thermal oxide films/CVD silicon nitride films/ALD-silicon oxide films405 serving as memory dielectric layers and phosphorus (P)-dopedpolysilicon films 406 are sequentially filled in the gaps, andplanarized by CMP. Similar to the third embodiment, this embodimentprocesses the gate electrodes by CMP, and hence requires nohigh-aspect-ratio RIE patterning.

As shown in FIG. 22, a plasma CVD silicon oxide film 407 serving as aninterlayer dielectric is formed on the entire substrate surface, andcontact plugs 408 communicating with the gate electrodes are formed.These contact plugs are formed into a zigzag pattern. Then,interconnections that connect the contact plugs are formed. Theseinterconnections connect alternate phosphorus (P)-doped polysiliconfilms 405 filled between the AA regions adjacent to each other in thehorizontal direction. That is, a dual-gate electrode structure in whichtwo independent gate electrodes sandwich one AA region is formed.

After that, similar to the first to third embodiments, contact plugsconnecting to the AA regions and gate electrodes (GCs) are formed, and aflash memory is formed through multilayered interconnection formationsteps. However, the details will not be explained.

In this embodiment, information can be independently written in the twothermal oxide films/CVD silicon nitride films/ALD-silicon oxide filmssandwiching the AA region. This makes it possible to write informationhaving one or more bits in one cell. For example, multilevel write of2×2=4 (i.e., two bits/cell) or 3×3=9 (i.e., three bits/cell) is possibleif a binary or ternary threshold value is written in the alumina/siliconnitride films/silicon oxide films on the two sides of the AA region.

Also, similar to the first to third embodiments, the structure of thisembodiment is tough against the short-channel effect and narrow-channeleffect.

As described above, the structure of the fourth embodiment can increasethe bit density without any special micropatterning, and does notincrease the number of lithography steps requiring the minimum halfpitch.

(Fifth Embodiment)

FIGS. 23 to 26 are perspective views showing a semiconductor memory(flash memory) according to the fifth embodiment of the presentinvention in predetermined main manufacturing steps.

This embodiment is directed to a four-layered memory similar to thesecond to fourth embodiments, but forms back gate electrodes forimproving the erase characteristic of memory cells.

First, as shown in FIG. 23, layered epitaxial films are formed byalternately stacking four epitaxial silicon germanium films and fourepitaxial silicon films 502 on a semiconductor substrate 501 followingthe same procedure as in the second to fourth embodiments, and aperipheral circuit portion is formed in the same manner as in the firstto fourth embodiments. Subsequently, following the same procedure as inthe second to fourth embodiments, a plasma CVD silicon oxide film 503, asilicon nitride film 504, and the layered epitaxial films in a cellregion are simultaneously processed into stripe patterns of AA regionsby using the conventional lithography technique and reactive ionetching, and only the epitaxial silicon germanium films of the layeredepitaxial films are removed by selective etching. The selective etchingis wet etching by mixture of fluoric acid and nitric acid or gas etchingwith CF₄/O₂ system. Then, SOG is formed by coating and changed into anoxide film under appropriate annealing conditions, thereby filling allthe spaces between the linear AA regions with SOG films 505. After that,back gate electrodes parallel to the AAs are formed. That is, theconventional lithography technique and RIE technique are used to etchback alternate SOG films 505 filled between the AAs, thereby forminggaps as templates of back gate electrodes. Since the processing isperformed on every other SOG film, no minimum half pitch patterning isnecessary.

As shown in FIG. 24, the end faces of the AA regions exposed by theprocessing are thermally oxidized, and the thermal oxide films areremoved by dry pre-treatment, thereby removing the process damage. Then,gate oxide films 506 are formed, and a P-doped polysilicon film 507 forforming gate electrodes is filled. The P-doped polysilicon film isrecessed to remain in only the gaps described above. Note that the upperportions of the P-doped polysilicon films are made lower than theuppermost surface of the substrate, so that the gaps remain above theP-doped polysilicon films.

Subsequently, as shown in FIG. 25, silicon nitride films 508 are filledin the gaps above the P-doped polysilicon films, and recessed by RIEagain. After that, templates of GC electrodes to be formed into stripesin a direction perpendicular to the AAs are formed. The conventionallithography and reactive ion etching are used to etch back the plasmaCVD silicon oxide films 503, further etch back every other SOG film 505in the gap between the AAs where no back gate electrode is formed, andfinally recess the silicon nitride films 508, thereby forming trenchesas templates of GC electrodes.

Then, as shown in FIG. 26, the end faces of the AA regions exposed inthe trenches by the processing are thermally oxidized, and the thermaloxide films are removed by dry preprocessing, thereby removing theprocess damage. Thermal oxide films/CVD silicon nitridefilms/ALD-hafnium aluminum oxide films 509 serving as memory dielectriclayers and P-doped polysilicon films 510 are sequentially formed.Subsequently, CMP is used to leave the P-doped polysilicon films in onlythe trenches.

After that, contact plugs connecting to the AA regions and GC electrodesare formed, and interlayer dielectrics, interconnections, and the likeare formed in multilayered interconnection formation steps, therebyforming a flash memory. However, the details will not be explained.

In this embodiment, the two gate electrodes sandwiching the AA regioncan be independently controlled, the gate electrode can be controlledwith respect to each cell, and a plurality of AA regions share the backgate electrode. This structure does not change the write characteristic.However, the structure improves the erase characteristic because it ispossible during data erase to forcedly remove electric charge from thehafnium aluminum oxide film/silicon nitride film/silicon oxide film byapplying an electric field to the back gate electrode.

Also, similar to the first to third embodiments, the structure of thisembodiment is tough against the short-channel effect and narrow-channeleffect.

As described above, the structure of the fifth embodiment can increasethe bit density without any special micropatterning, and does notincrease the number of lithography steps requiring the minimum halfpitch.

It is obvious that the effects of the present invention do not reduceeven when the memory dielectric layers, gate electrode formationmethods, contact plug formation methods, and the like described in thefirst to fifth embodiments are used in different combinations.

The differences between the structures of the first to fifth embodimentswill be compared below with reference to FIGS. 27 to 29.

The present invention basically has three structures different in GCelectrode arrangement, and each structure is identifiable by the sectionin a plane perpendicular to AA regions. The AAs are formed byalternately stacking epitaxial silicon films 601 and insulating films602.

As shown in FIG. 27, the first to third embodiments are directed to adouble-gate structure in which gate electrodes 604 connected to eachother and having the same potential sandwich each AA region via a memorydielectric layer 603.

As shown in FIG. 28, the fourth embodiment is directed to a dual-gatestructure in which two types of gate electrodes 604 and 605 that areindependently controllable sandwich each AA region via a memorydielectric layer 603. Note that FIG. 28 shows two different sections inorder to illustrate zigzag interconnections to the gate electrodes.

As shown in FIG. 29, the fifth embodiment is directed to a dual-gatestructure in which back gate electrodes 607 parallel to AA regions arefilled between the AA regions via gate insulating films 605, gateelectrodes 604 perpendicular to the AA regions are connected to eachother, and the gate electrode 604 and back gate electrode 607 sandwicheach AA region.

Also, to avoid the complexity of explanation, the first to fifthembodiments have shown the forms of two- and four-layered memories.However, the methods of these embodiments are evidently applicable tomemories having larger numbers of layers. The use of these embodimentsmakes it possible to continuously increase the degree of integration ofmemories in the future, so various application fields presumably extend.

In the above embodiments, the stacked AA regions share the sourcecontact, and are connected to the peripheral circuit by individuallyforming the drain contacts. In embodiments to be explained below,however, each AA region is selected on the source side.

(Sixth Embodiment)

Each of FIGS. 30 to 40 is a sectional or perspective view showing asemiconductor memory (flash memory) according to the sixth embodiment ofthe present invention in a predetermined main fabrication step.

This embodiment is directed to a four-layered stacked memory similar tothe second to fourth embodiments. However, this embodiment usespolysilicon films obtained by crystallizing amorphous silicon films asAA regions, and uses layer selection transistors that select individuallayers.

First, as shown in FIG. 30, four silicon oxide films 702 and fouramorphous silicon films are alternately stacked on a semiconductorsubstrate 701, and a silicon nitride film 703 and a silicon oxide film704 serving as a hard mask are formed on top of the structure. Then, theamorphous silicon films are crystallized by annealing to formpolysilicon films 705. Similar to the first to fifth embodiments, thestacked films are removed from a peripheral circuit portion, and B-dopedpolysilicon films 706 are formed on the sidewalls of the stacked films,thereby connecting the polysilicon films 705. Transistors 707 forming aperipheral circuit are formed on the surface of the semiconductorsubstrate 701 exposed by removing the stacked films. The obtainedstructure is planarized by filling an interlayer dielectric 708.

Then, as shown in FIGS. 31 and 32, the conventional lithographytechnique and RIE technique are used to simultaneously process thestacked silicon oxide film 704, silicon nitride film 703, silicon oxidefilms 702, and polysilicon films 705 into stripe patterns of AA regions,thereby forming stacked AAs. The side surfaces of the polysilicon filmsare thermally oxidized, the thermal oxide films are removed by drypre-treatment, and silicon thermal oxide films/CVD-silicon nitridefilms/ALD-alumina films 709 serving as memory dielectric layers aresequentially formed. Subsequently, P-doped polysilicon films serving asgate electrodes are filled and processed by using the conventionallithography technique and RIE technique, thereby forming gate electrodes710 that the stacked AAs share. These gate electrodes are used as masksto ion-implant As by the conventional ion implantation technique. Afterthat, the sidewalls of the gate electrodes are oxidized to activatediffusion layers and form extension regions.

In this embodiment, it is possible that it is used as a cell transistorof depletion type. For example, instead of forming the source/drain areaby doping, it dopes impurities such as P at the time of depositing thepolysilicon films 705 or it dopes impurities such as P after forming thepolysilicon films 705. By this process, impurities are doped in thepolysilicon films 705 of the cell region uniformly, the depletion layerwhich appears by applying a voltage to the gate electrode is used forON/Off operation of the cell transistor.

As shown in FIGS. 33 and 34, an SOG film is filled between the gateelectrodes and converted into a silicon oxide film, thereby forming aninterlayer dielectric 711. The conventional lithography technique andRIE technique are used to expose the first and third polysilicon films705.

Subsequently, as shown in FIGS. 35 and 36, the conventional lithographytechnique and RIE technique are used to expose the second and fourthpolysilicon films 705.

As shown in FIGS. 37 and 38, the entire substrate surface is etched backto expose the surfaces and side surfaces of the polysilicon films 705.After that, gate oxide films/gate electrode films are formed andpatterned by the conventional lithography technique and RIE technique,thereby forming layer selection gate transistors 712 for selectinglayers. The formation of the layer selection gate transistors forms atri-gate structure in which gate electrodes exist in three directions ofthe channel region. This increases the controllability of the gates andimproves the cutoff performance.

As shown in FIG. 39, an interlayer dielectric 713 is formed on theentire substrate surface, contact holes connecting to the AA regions,gate electrodes, and peripheral circuit are formed, and a conductor filmis filled, thereby forming contact plugs 714 on the source and drainsides, interconnections 715 between the layer selection gates andsource-side contacts and between the drain-side contacts and peripheralcircuit, and word line contact plugs connecting to the gate electrodes.

After that, as shown in FIG. 40, a flash memory is formed by forminginterlayer dielectrics 716, 717, and 718, contact plugs 719 and 720, andinterconnections 721 and 722 in multilayered interconnection formationsteps. However, the details will not be explained.

To perform data write and read in this embodiment, an arbitrary cell canbe selected by first selecting one of the first to fourth layers by thelayer gate transistor, selecting an AA region in the layer by thedrain-side contact plug, and then selecting a gate electrode. In theconventional NAND flash memory, an arbitrary cell can be selected byselecting two AA regions sharing a source contact by a select gatetransistor, selecting a desired AA region from a plurality of AA regionssharing the select gate electrode by a drain-side contact plug, andselecting a gate electrode. Therefore, the cell selecting operation issubstantially the same as that of this embodiment. This makes itpossible to implement the peripheral circuit and the like by a circuitconfiguration similar to that of the conventional NAND flash memory.That is, this embodiment has high affinity for the design of theconventional NAND flash memory, compared to the method of extracting thedrain-side contact for each layer disclosed in the first to fifthembodiments.

Note that this embodiment uses the polysilicon film as each AA region,but each AA region can also be formed by a single-crystal silicon filmin the same manner as described in the first to fifth embodiments.

Select gates 723 as used in the conventional NAND flash memory may alsobe formed in a cell structure similar to that of this embodiment. FIG.41 shows an example.

In this structure, the select gate 723 selects one of stacked NAND chainarrays, the layer selection gate transistor 712 selects one of the firstto fourth layers, the drain-side contact plug 714 selects an AA regionin the layer, and the gate electrode 710 selects a cell in the same AAregion.

Note that the select gates allow the above-mentioned layer selectiongate transistors to be simultaneously formed, and high cutoffperformance is readily achieved by forming the select gates on thesemiconductor substrate.

The effect of suppressing the increase in number of steps that thisembodiment has on the prior art will be explained below. Sinceestimating all steps is too complicated, the numbers of lithographysteps requiring the highest process cost will be compared.

The number of times of lithography necessary to form the four-layeredmemory of this embodiment by the method of sequentially forming theindividual layers according to the prior art is as follows:

(1) The number of times of lithography per layer: 3, i.e., 2 for AAprocessing and gate electrode (GC: Gate Conductor) processing, and 1 forformation of contact plugs between layers

(2) Formation of contacts to stacked AAs: 1

Accordingly, a total of 3×4+1=13 lithography steps are necessary.

By contrast, this embodiment requires:

(1) Simultaneous processing of stacked AAs: 1

(2) Simultaneous processing of GCs: 1

(3) Processing for exposing end portions of stacked AA regions in orderto form contact holes: 3

(4) Layer selection gate processing: 1

(5) Contact hole formation and connection of layer selection gates andsource lines: 2

That is, a total of seven lithography steps can implement theembodiment.

The layer selection gates also have the following advantage in additionto implementing a stacked memory without largely increasing the numberof times of lithography as described above.

When extracting the contact plugs from the stacked AA regions, it isdifficult to extract interconnections parallel to the AA regions becausethe interval between adjacent AA regions is normally the minimum halfpitch in the memory. Therefore, interconnections must be extractedperpendicularly to the AA regions. For this purpose, it is necessary towiden the intervals between the contact plugs arranged parallel to theAA regions so as to allow the interconnections to run between thecontact plugs. This makes it difficult to increase the bit density.

By contrast, when the layer selection gate transistors are formed as inthis embodiment, the numbers of contacts on the source and drain sidesof the AA regions are the same as in the conventional unstacked NANDflash memory, and the connection to the peripheral circuit is also thesame as in the conventional memory. The gate electrode that drives eachlayer selection gate transistor is extracted parallel to the controlgate electrode of each cell. Therefore, the method of connecting thegate electrodes and the peripheral circuit is also basically the same asin the conventional unstacked NAND flash memory. That is, the number ofselect gates that originally each exist for one NAND chain increases inaccordance with the number of stacked layers. This results in the bigadvantage that the peripheral circuit need not be largely changed.

(Seventh Embodiment)

FIGS. 42 to 52 are sectional views showing a semiconductor memory (flashmemory) according to the seventh embodiment of the present invention inpredetermined main manufacturing steps.

This embodiment is an example in which single-crystal silicon layers arestacked as in the first to fifth embodiments. In this example, however,a memory cell region is dug down beforehand in order to increase theflatness of the structure. Also, dummy gate electrodes are used to formdiffusion layers.

First, as shown in FIG. 42, a silicon oxide film 802 as a hard mask isformed on a semiconductor substrate 801, and a memory cell region is dugdown by the conventional lithography technique and RIE technique. Inaddition, a plasma CVD silicon oxide film 803 is formed on the entiresurface, and left behind on only the sidewalls of the dug region by RIEthat leaves sidewalls behind.

Then, as shown in FIG. 43, eight epitaxial silicon germanium films 804and eight epitaxial silicon films 805 are selectively sequentiallystacked. In addition, a P-doped polysilicon film 806 is formed on theentire substrate surface, and left behind on only the sidewalls of thestacked epitaxial films by RIE etch back. A silicon nitride film 807 anda silicon oxide film 808 serving as a hard mask are formed on top of thestructure. After that, transistors 809 forming a peripheral circuit areformed on the surface of the semiconductor substrate 801 except for thecell region, and the obtained structure is planarized by filling aninterlayer dielectric 810.

Then, as shown in FIG. 44, the conventional lithography technique andRIE technique are used to simultaneously process the stacked siliconoxide film 808, silicon nitride film 807, epitaxial silicon films 805,and epitaxial silicon germanium films 804 into stripe patterns of AAregions. The epitaxial silicon germanium films 802 are selectivelyremoved by wet etching. Subsequently, the entire substrate surface iscoated with SOG, and SOG films 811 are filled in gaps formed by theselective wet etching of the silicon germanium films, thereby formingstacked AAs. Furthermore, the SCG films between the stacked AAs arrayedinto stripes are etched back by RIE, thereby exposing the side surfacesof the epitaxial silicon films 805 forming the AAs.

As shown in FIG. 45, the side surfaces of the epitaxial silicon films803 are thermally oxidized to form silicon thermal oxide films, and anamorphous silicon film for forming dummy gate electrodes is formed.Subsequently, the conventional lithography technique and RIE techniqueare used to process the amorphous silicon film, thereby forming dummygate electrodes 812.

Subsequently, as shown in FIG. 46, the dummy gate electrodes are used asmasks to remove the silicon thermal oxide films by dry pre-treatment.After that, As-doped silicon oxide films 813 are filled by using LPCVD,and diffusion layers are formed by contact diffusion. The dummy gateelectrodes 812 are then removed by chemical dry etching (CDE), and thesilicon thermal oxide films are removed by dry pre-treatment, therebyforming gaps as templates of gate electrodes.

As shown in FIG. 47, thermal oxide films/CVD-silicon nitridefilms/ALD-hafnium oxide films 814 serving as memory dielectric layersare sequentially formed. Then, CVD-TiN films serving as gate electrodesare filled and processed by CMP, thereby forming gate electrodes 815that the stacked AA regions share.

As shown in FIG. 48, an interlayer dielectric 816 is formed. Theconventional lithography technique and RIE technique are used to exposethe eighth and fourth epitaxial silicon films 805.

As shown in FIG. 49, the conventional lithography technique and RIEtechnique are used to expose the sixth and second epitaxial siliconfilms 805.

As shown in FIG. 50, the conventional lithography technique and RIEtechnique are used to expose the seventh, fifth, third, and firstepitaxial silicon films 805. Then, the entire substrate surface isetched back to expose the surfaces and side surfaces of the epitaxialsilicon films 805. After that, gate oxide films/gate electrode films areformed and processed by the conventional lithography technique and RIEtechnique, thereby forming layer selection gate transistors 817 forselecting layers.

As shown in FIG. 51, an interlayer dielectric 818 is formed on theentire substrate surface, contact holes connecting to the AA regions,gate electrodes, and peripheral circuit are formed, and a conductor filmis filled, thereby forming contact plugs 819 on the source and drainsides, interconnections 820 that connect the layer selection gatetransistors and source-side contacts, and word line contact plugsconnecting to the gate electrodes.

After that, as shown in FIG. 52, a flash memory is formed by forminginterlayer dielectrics 821, 822, and 823, contact plugs 824 and 825, andinterconnections 826 and 827 in multilayered interconnection formationsteps. However, the details will not be explained.

This embodiment recesses the stacked cell portion to be lower than thesurface of the semiconductor substrate. This suppresses the substratestep when forming the multilayered interconnection, and facilitates thelithography steps and processing steps. This embodiment also has theadvantage that the diffusion layers can be easily formed by using thedummy gate electrodes.

The effect of suppressing the increase in number of steps that thepresent invention has on the prior art will be explained below in thisembodiment as well. Since estimating all steps is too complicated, thenumbers of lithography steps requiring the highest process cost will becompared.

The number of times of lithography necessary to form the eight-layeredmemory of this embodiment by the method of sequentially forming theindividual layers according to the prior art is as follows:

(1) The umber of times of lithography per layer: 3, i.e., 2 for AAprocessing and gate electrode (GC: Gate Conductor) processing, and 1 forformation of contact plugs between layers

(2) Formation of contacts to stacked AAs: 1

Accordingly, a total of 3×8+1=25 lithography steps are necessary.

By contrast, this embodiment requires:

(1) Processing for digging down a cell region: 1

(2) Simultaneous processing of stacked AAs: 1

(3) Simultaneous processing of GCs: 1

(4) Processing for exposing end portions of stacked AAs in order to formcontact holes: 3

(5) Layer selection gate processing: 1

(6) Contact hole formation and connection of layer selection gates andsource lines: 2

Accordingly, a total of nine lithography steps can implement theembodiment. That is, the use of the present invention makes it possibleto reduce the number of lithography steps to almost ⅓.

Each of the above embodiments has explained an example in which a planarMONOS memory widespread as a nonvolatile memory is used in a form inwhich a channel is formed parallel to a semiconductor substrate in aplane perpendicular to the substrate. However, the above embodiments arealso effective in nonvolatile memories using different cell structuresand different storage principles. Even in these cases, the contact orlayer selection gate formation method remains the same. Therefore, anexplanation will be limited to the cell portion formation methodhereinafter in order avoid complexity.

(Eighth Embodiment)

FIGS. 53 to 55 are sectional views showing a semiconductor memory (flashmemory) according to the eighth embodiment of the present invention inpredetermined main manufacturing steps.

This embodiment is an example of a MONOS memory similar to the first toseventh embodiments, but gives the channel surface a curvature in orderto improve the write/erase characteristics.

Stacked AA regions are formed in a memory cell portion in the samemanner as in the embodiments explained above. For the sake ofsimplicity, an explanation will be made with reference to only sectionalviews of the AAs.

First, as shown in FIG. 53, in a memory cell region on a semiconductorsubstrate 901, epitaxial silicon films 902 serving as AA regions andsilicon oxide films 903 that separate AA regions are stacked as they areprocessed by using hard masks 904 into a shape in which AA regions aresimultaneously processed into stripes.

Then, as shown in FIG. 54, the exposed side surfaces of the epitaxialsilicon films 902 are oxidized by steam oxidation, and bird's beakoxidation is caused by steam diffused in the silicon oxide films 903,thereby forming silicon thermal oxide films 905 such that the sidesurfaces of the epitaxial silicon films 902 processed into stripes havea sectional shape with a curvature.

As shown in FIG. 55, the silicon thermal oxide films 905 are removed bydry pre-treatment, and the silicon oxide films 903 are partiallyrecessed, thereby forming a shape in which the epitaxial silicon films902 whose side surfaces have a curvature are stacked. Subsequently, theentire substrate surface is coated with SOG to fill SOG films 906 ingaps between the stacked AA regions. The conventional lithographytechnique and wet etching technique are used to form gaps as templatesof gate electrodes by etch back. Then, thermal oxide films/siliconnitride films/silicon oxide films 907 are sequentially formed, andP-doped polysilicon films 908 are filled and processed by CMP, therebyforming MONOS memory cells.

In this embodiment, the channel of each of the AA regions verticallyarranged parallel to the substrate is formed into a shape having acurvature. Since electric field concentration raises the effectiveelectric field, a memory cell operation at a lower write/erase voltagecan be implemented.

(Ninth Embodiment)

FIGS. 56 to 58 are sectional views showing a semiconductor memory (flashmemory) according to the ninth embodiment of the present invention inpredetermined main manufacturing steps.

Unlike the first to eighth embodiments, this embodiment is an example inwhich the present invention is applied to a phase change RAM (PRAM).Similar to a resistive RAM (RRAM) to be described later, the PRAM sensesthe change in resistance of a memory element by the change in amount ofan electric current flowing through the memory element. Accordingly, itis unnecessary to stack transistors as in a MONOS memory, but diodesneed to be stacked.

Similar to the previous embodiments, this embodiment also forms stackedAA regions in a memory cell portion. An explanation will be made withreference to sectional views of the AA regions.

First, as shown in FIG. 56, in a memory cell region on a semiconductorsubstrate 1001, B-doped polysilicon films 1003 serving as AA regions andsilicon oxide films 1004 that separate AA regions are stacked into ashape in which AA regions are simultaneously processed into stripes byhard masks 1002. Interlayer dielectrics 1005 are filled between thestacked AA regions.

Then, as shown in FIG. 57, the conventional lithography technique andRIE technique are used to form gaps as templates of diodes and memoryelements. These gaps are formed in alternate spaces between the AAregions arranged into stripes for the reason explained below. That is,in a memory such as a PRAM or RRAM that senses a resistance change, evenwhen elements are formed at the two ends of one AA, if the resistance ofthe element at one end lowers due to data write, it becomes difficult toapply a voltage to the element at the other end. Consequently, thefollowing three states:

(1) The state in which the resistances of the elements at the two endslower

(2) The state in which the resistance of the element at the right endlowers

(3) The state in which the resistance of the element at the left endlowers

and intermediate states (because the element resistance of a PRAM orRRAM continuously changes) of these states coexist. This varies thethreshold value of the memory operation.

As shown in FIG. 58, gaps are formed by selectively and isotropicallyrecessing the B-doped polysilicon films 1003 by chemical dry etching. AP-doped polysilicon film 1006 is formed on the entire substrate surface,and left behind in only the gaps in the B-doped polysilicon films 1003by non-masking RIE etch back, thereby forming diode portions. TheP-doped polysilicon films are then recessed by chemical dry etching.Subsequently, tungsten films 1007 serving as heaters are selectivelygrown on the surfaces of the recessed P-doped polysilicon films by usingCVD. After that, GST films (Ge₂Sb₂Te₅) 1008 and CVD titanium nitridefilms 1009 serving as electrode films are formed by using CVD, andprocessed by using the CMP technique, thereby forming PRAM memory cells.

This embodiment uses the method of filling an n-type semiconductor in ap-type semiconductor as the diode formation method. As shown in FIG. 59,however, it is also possible to form n-type semiconductor layers 1010 bydiffusing an impurity on the surfaces of the gaps formed by selectivelyand isotropically recessing the B-doped polysilicon films 1003, andpartially filling the GST films 1008 in the recesses of the tungstenfilms 1007 as heater films, thereby improving the write characteristic(reducing the write voltage).

(10th Embodiment)

FIGS. 60 to 62 are sectional views showing a semiconductor memory (flashmemory) according to the 10th embodiment of the present invention inpredetermined main manufacturing steps.

This embodiment is an example in which the present invention is appliedto a resistive RAM (RRAM).

Similar to the previous embodiments, this embodiment also forms stackedAA regions in a memory cell portion. An explanation will be made withreference to sectional views of the AA regions.

First, as shown in FIG. 60, in a memory cell region on a semiconductorsubstrate 1101, B-doped polysilicon films 1103 serving as AA regions andsilicon oxide films 1104 that separate AA regions are stacked into ashape in which AA regions are simultaneously patterned into stripes byusing hard masks 1102. Interlayer dielectrics 1105 are filled betweenthe stacked AA regions.

Then, as shown in FIG. 61, the conventional lithography technique andRIE technique are used to form gaps as templates of diodes and memoryelements. These gaps are formed in alternate spaces between the AAregions arranged into stripes. The purpose is the same as described inthe ninth embodiment. Subsequently, gaps are formed by selectively andisotropically recessing the B-doped polysilicon films 1103 by chemicaldry etching. A P-doped polysilicon film 1106 is formed on the entiresubstrate surface, and left behind in only the gaps in the B-dopedpolysilicon films 1103 by non-masking RIE etch back, thereby formingdiode portions. The P-doped polysilicon films 1106 are then recessed bychemical dry etching. A CVD titanium nitride film 1107 and rutheniumfilm 1108 are formed by using CVD, and etched back by using theconventional RIE technique so as to remain in only the gaps formed byrecessing the P-doped polysilicon films.

Subsequently, as shown in FIG. 62, a zirconia film 1109 for formingresistive elements is formed on the entire substrate surface, and aruthenium film 1110 for forming upper electrodes is formed by using ADL.These films are processed by using the conventional CMP technique,thereby forming RRAM memory cells.

This embodiment uses the method of filling an n-type semiconductor in ap-type semiconductor as the diode formation method. As described in theninth embodiment, however, diodes may also be formed by forming n-typesemiconductor layers by diffusing an impurity on the surfaces of thegaps formed by selectively and isotropically recessing the B-dopedpolysilicon films 1102.

As the insulating film serving as a resistive element, it is alsopossible to use, e.g., a titania film, hafnia film, or nickel oxidefilm, instead of a zirconia film. It is clear that the effect (ofincreasing the bit density per unit area) of this embodiment is obtainedregardless of whether any of these films is used.

(11th Embodiment)

FIGS. 63 to 65 are sectional views showing a semiconductor memory (flashmemory) according to the 11th embodiment of the present invention inpredetermined main manufacturing steps.

Similar to the 10th embodiment, this embodiment is also an example inwhich the present invention is applied to a resistive RAM (RRAM).However, a resistive element is selectively formed by using the platingtechnique.

Similar to the previous embodiments, this embodiment also forms stackedAA regions in a memory cell portion. An explanation will be made withreference to sectional views of the AA regions.

First, as shown in FIG. 63, similar to the 10th embodiment, in a memorycell region on a semiconductor substrate 1201, B-doped polysilicon films1203 serving as AA regions and silicon oxide films 1204 that separate AAregions are stacked into a shape in which AA regions are simultaneouslyprocessed into stripes by using hard masks 1202. Interlayer dielectrics1205 are filled between the stacked AA regions.

Then, as shown in FIG. 64, the conventional lithography technique andRIE technique are used to form gaps as templates of diodes and memoryelements. These gaps are formed in alternate spaces between the AAregions arranged into stripes. The purpose is the same as described inthe ninth embodiment. Subsequently, gaps are formed by selectively andisotropically recessing the B-doped polysilicon films 1203 by chemicaldry etching. Diodes are formed by forming n-type semiconductor layers1206 by diffusing P by gas phase doping (GPD) on the surfaces of thegaps formed by selectively and isotropically recessing the B-dopedpolysilicon films 1203. A CVD titanium nitride film 1207 and rutheniumfilm 1208 are formed by using CVD, and etched back by using theconventional RIE technique so as to remain in only the gaps formed inthe B-doped polysilicon films 1203.

Subsequently, as shown in FIG. 65, nickel films are selectively formedon the surfaces of ruthenium by using electroplating (EP), and platinumfilms 1209 are also selectively formed by using the EP process, therebycompletely filling the gaps formed between the AA regions. This formsupper electrodes of RRAM elements. At this point, however, no resistiveelements are formed. Then, oxidation is performed in oxygen at 500° C.The platinum films 1209 do not oxidize but transmit oxygen. Since a lowtemperature of 500° C. is selected, however, this oxidation processoxidizes the nickel films without oxidizing the ruthenium films 1208 aslower electrodes, thereby forming nickel oxide films 1210 as resistiveelements. In this manner, RRAM memory cells are formed.

Although the first to 11th embodiments of the present invention havebeen explained above, forms and methods of practicing the presentinvention are not limited to these embodiments. That is, it is possibleto appropriately combine the AA region formation methods, gate electrodeprocessing methods, contact hole formation methods, layer selection gateformation methods, materials, and the like disclosed in the embodiments.Even in these cases, it is possible to achieve the effect expected fromthe embodiments of the present invention, i.e., increase the degree ofintegration per unit area without largely increasing the number ofmanufacturing steps. Accordingly, a high density memory can bemanufactured relatively easily.

In addition, the above embodiments include the following modes.

(1) A semiconductor memory is characterized by comprising stripe-likeactive areas (AAs) stacked parallel to a substrate, wherein the AAsstacked perpendicularly to the substrate are patterned in self-alignmentwith each other, each AA uses, as a channel region, one or both of theside surfaces perpendicular to the substrate, and intersects a pluralityof gate electrodes (GCs) in the longitudinal direction, theintersections of the AAs and GCs form memory cells, and a plurality ofcells in an intersecting plane share the gate electrode.

(Effects of Mode (1))

(a) The bit density per unit area can be increased by stacking thememory cells. That is, the degree of integration can be increasedwithout any micropatterning.

(b) Since an SOI structure is formed, a transistor tough against theshort-channel effect can be implemented.

(c) The structure is basically a double-gate electrode structure. Thisimproves the controllability of a transistor.

(2) A semiconductor memory is characterized by comprising stripe-likeactive areas (AAs) stacked parallel to a substrate, wherein the AAsstacked perpendicularly to the substrate are patterned in self-alignmentwith each other, a memory dielectric layer and gate electrode are formedon a side surface, which is perpendicular to the substrate, of each AA,each AA intersects a plurality of gate electrodes (GCs) in thelongitudinal direction, the intersections of the AAs and GCs form memorycells, each cell in an intersecting plane shares the gate electrode withan adjacent cell, and gate electrodes sandwiching each AA are connectedby interconnections so as to be driven independently of each other.

(Effect of Mode (2))

(a) Since two independent gate electrodes are formed for one cell,multilevel storage is possible. This makes it possible to increase thebit density without any special micropatterning.

(3) A semiconductor memory is characterized by comprising stripe-likeactive areas (AAs) stacked parallel to a substrate, wherein the AAsstacked perpendicularly to the substrate are processed in self-alignmentwith each other, a memory dielectric layer and gate electrode are formedon a side surface, which is perpendicular to the substrate, of each AA,each AA intersects a plurality of gate electrodes (GCs) in thelongitudinal direction, the intersections of the AAs and GCs form memorycells, each cell has a structure sandwiched between first and secondgate electrodes, cells in a plane perpendicular to the AAs share thefirst gate electrode, and the second gate electrode is formed parallelto the AAs and shared by a plurality of cells in a plane parallel to theAA direction and perpendicular to the substrate.

(Effect of Mode (3))

(a) Since a back gate electrode is formed for each AA, electric chargewritten in the memory dielectric layer can be readily removed in dataerase. That is, the erase characteristic improves.

(4) A semiconductor memory is characterized by comprising stripe-likeactive areas (AAs) stacked parallel to a substrate, wherein the AAsstacked perpendicularly to the substrate are patterned in self-alignmentwith each other, a memory layer and gate electrode are formed on asidesurface, which is perpendicular to the substrate, of each AA, each AAintersects a plurality of gate electrodes (GCs) in the longitudinaldirection, the intersections of the AAs and GCs form memory cells, eachcell has a structure sandwiched between an insulating film and the gateelectrode, and a plurality of cells in a plane parallel to the AAdirection and perpendicular to the substrate share the gate electrode.

(Effect of Mode (4))

(a) When memory layers sharing a gate electrode are formed on the twosides of an AA in a cell such as a PRAM cell or RRAM cell that sensesthe change in current amount by the change in resistance of a memorylayer, if one memory layer is turned on and the resistance lowers, noelectric current flows through the other memory layer having a highresistance. This makes the other memory layer difficult to turn on.However, the memory layers on the two sides of the AA are sometimesturned on in a certain cell. This makes it difficult to set the state ofeach cell constant. Since the electrode is formed on every other row,each AA is controlled by only the gate electrode on one side. Thiseliminates the instability of the write/erase characteristics.

(Effect of Mode (4) & (5))

(a) the cell transistor uses the depletion-type.

By this, the source/drain of the transistor is not formed in particular.This facilitates transistor shrinkage.

(5) A semiconductor memory described in any one of modes (1) to (4) ischaracterized by comprising stripe-like active areas (AAs) stackedparallel to a substrate, and gate electrodes (GCs) shared by the AAs,wherein each AA is made of single-crystal silicon or polysilicon.

(6) A semiconductor memory described in any one of modes (1) to (5) ischaracterized by comprising stripe-like active areas (AAs) stackedparallel to a substrate, gate electrodes (GCs) shared by the AAs, and alayer selection gate transistor for selecting a layer to which a groupof AAs, among other AAs arranged into stripes, in a plane parallel tothe substrate belongs.

(Effects of Mode (6))

(a) The use of the layer selection gate transistor obviates the need toextract interconnections from the stacked AAs in the directionperpendicular to them, and makes it possible to extract all theinterconnections by the minimum pitch. This facilitates increasing thebit density.

(b) Peripheral circuits can be basically the same as in the conventionalNAND flash memories. This facilitates circuit design.

(c) In the forming the layered select gate transistor which is selectedeach layers of stripe-like active area (AA) stacked parallel to asubstrate, it is not necessary to increase the large number of processsteps in this case that lithography process to expose each AA in thestairs-shaped, etching process and forming process the layer select gatetransistor in each AA by simultaneous patterning. lithography process toexpose AA for layer selection gate transistor formation is three timesin 4 layer lamination, four times in 8 layer lamination, n+1 times in2^(n) layer lamination.

(7) A semiconductor memory described in any one of modes (1) to (6) ischaracterized by comprising stripe-like active areas (AAs) stackedparallel to a substrate, and gate electrodes (GCs) shared by the AAs,wherein each of the stacked AAs is formed in a recess dug down from thesubstrate surface.

(Effect of Mode (7))

(a) Since the memory cell portion is formed in the recess dug down fromthe substrate surface, difference in level on the substrate surface canbe suppressed. This facilitates lithography steps, processing steps, andinterconnection formation.

(8) A semiconductor memory described in any one of modes (1) to (3) and(5) to (7) is characterized by comprising stripe-like active areas (AAs)stacked parallel to a substrate, and gate electrodes (GCs) shared by theAAs, wherein a memory element is a MONOS(Metal-Oxide-Nitride-Oxide-Silicon) element.

(9) A semiconductor memory described in mode (8) is characterized bycomprising stripe-like active areas (AAs) stacked parallel to asubstrate, gate electrodes (GCs) shared by the AAs, wherein a memoryelement is a MONOS (Metal-Oxide-Nitride-Oxide-Silicon) element, and theshape of the AA sidewall on which the memory element is formed has acurvature outward.

(Effect of Mode (9))

(a) Since the channel has a curvature, electric field concentrationraises the effective electric field. This makes it possible to improvethe write/erase characteristics.

(10) A semiconductor memory described in any one of modes (1) and (4) to(7) is characterized by comprising stripe-like active areas (AAs)stacked parallel to a substrate, and gate electrodes (GCs) shared by theAAs, wherein a memory element is a PRAM (Phase Change RAM).

(11) A semiconductor memory described in any one of modes (1) and (4) to(7) is characterized by comprising stripe-like active areas (AAs)stacked parallel to a substrate, and gate electrodes (GCs) shared by theAAs, wherein a memory element is an RRAM (Resistive RAM).

(12) A semiconductor memory manufacturing method which has stripe-likeactive areas (AAs) stacked parallel to a substrate described in mode(1), and forms the AAs stacked perpendicularly to the substrate inself-alignment with each other is characterized by alternately stackinginsulating films and epitaxial silicon films or polysilicon films on asemiconductor substrate, and simultaneously processing the stacked filmsinto AA shapes.

(13) A semiconductor memory manufacturing method which has stripe-likeactive areas (AAs) stacked parallel to a substrate described in mode(1), and forms the AAs stacked perpendicularly to the substrate inself-alignment with each other is characterized by alternately stackingepitaxial silicon germanium films and epitaxial silicon films on asemiconductor substrate, simultaneously processing the stacked filmsinto AA shapes, removing the epitaxial silicon germanium films by wetetching, and filling insulating films in the gaps.

(Effect of Modes (12) & (13))

(a) Since the AAs are simultaneously processed, each of AA lithographyand GC lithography requiring minimum half pitch patterning need only beperformed once regardless of the number of stacked memory layers.

(Effects of Mode (13))

(a) In AA processing, it is only necessary to process the stacked filmsmade of similar materials, i.e., epitaxial silicon and epitaxial silicongermanium. This facilitates RIE.

(b) Each epitaxial silicon layer as an AA is formed by epitaxial growth.This makes it possible to form a channel region of a transistor havinggood crystallinity without any complicated procedure such as solid-phaseepitaxial growth.

(14) A semiconductor memory manufacturing method of forming gateelectrodes that intersect stripe-like active areas (AAs) stackedparallel to a substrate described in mode (1) is characterized byfilling conductor films in gaps between the stripe-like active areas(AAs) stacked parallel to the substrate and insulating films formedbetween the AAs, and patterning the conductor films by reactive ionetching.

(15) A semiconductor memory manufacturing method of forming gateelectrodes that intersect stripe-like active areas (AAs) stackedparallel to a substrate described in mode (1) is characterized byfilling insulator films in gaps between the stripe-like active areas(AAs) stacked parallel to the substrate and insulating films formedbetween the AAs, forming gaps as templates of gate electrodes, fillingmemory dielectric layers and conductor films in the gaps, and patterningthe filled films by CMP.

(Effect of Modes (14) & (15))

(a) Since the GCs are simultaneously processed, each of AA lithographyand GC lithography requiring minimum half pitch patterning need only beperformed once regardless of the number of stacked memory layers.

(Effect of Mode (15))

(a) The GCs can be formed by a metal that is difficult to be processedby RIE.

(16) A semiconductor memory manufacturing method of forming contactplugs connecting to stripe-like active areas (AAs) stacked parallel to asubstrate described in mode (1) is characterized by arranging contactplugs at two ends of each active area, each of which is formed over twoadjacent AAs, so as to be in zigzag alignment with each other by oneactive area.

(Effect of Mode (16))

(a) Especially when two layers are stacked, contact plugs equal innumber to AAs in the same plane are formed at the two ends of the AAs.Therefore, it is possible to access each AA by selecting two contactplugs. This facilitates formation of interconnections.

(17) A semiconductor memory manufacturing method of forming diodes instripe-like active areas (AAs) stacked parallel to a substrate describedin mode (1) is characterized by comprising steps of filling insulatingfilms between rows of the stacked AAs, forming gaps as templates of gateelectrodes in the insulating films, forming recesses in the AA sidewallsby using the insulating films as masks, and forming diodes by filling,in the recesses, semiconductor films having a conductivity typedifferent from the AAs or by forming, on the surfaces of the recesses,semiconductor layers having a conductivity type different from the AAsby gas phase doping.

(Effect of Modes (17))

(a) It is possible to stack layers in a nonvolatile memory such as anRRAM or PRAM that senses the change in resistance.

(18) A semiconductor memory manufacturing method of forming electrodesof, e.g., an RRAM or PRAM in stripe-like active areas (AAs) stackedparallel to a substrate described in mode (1) is characterized bycomprising steps of filling insulating films between rows of the stackedAAs, forming gaps as templates of gate electrodes in the insulatingfilms, forming recessed diodes filled in the AA sidewalls by using theinsulating films as masks, and filling electrode films in the recesses.

(19) A semiconductor memory manufacturing method of forming an RRAMdescribed in mode (18) is characterized by comprising steps of fillingfirst electrode films in recessed diodes filled in the sidewalls ofstacked AAs, and forming memory dielectric films and second electrodefilms between rows of the AAs.

(Effects of Modes (17)-(19))

(a) It is possible to stack layers in a nonvolatile memory such as anRRAM or PRAM that senses the change in resistance.

(b) It is unnecessary to process the memory layer (a chalcogenide filmthat largely changes the resistance (normally, increases the resistanceby thousand times) when heated in the case of a PRAM, and a metal oxidefilm such as TiO₂, HfO₂, ZrO₂, or nickel oxide in the case of an RRAM).This makes RIE processing of any new material unnecessary.

As described above, the embodiments of the present invention can providea semiconductor memory having a structure in which memory layers can bestacked without largely increasing the number of process steps, andprovide a method of manufacturing the semiconductor memory. In addition,the semiconductor memories disclosed in the embodiments make it possibleto continuously increase the degree of integration of semiconductormemories, particularly, flash memories in the future. Accordingly, therange of applications of flash memories presumably further extends inthe future.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader modes is notlimited to the specific details and representative embodiments shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general inventive concept asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A semiconductor memory, comprising: a pluralityof insulating layers stacked above a substrate; a plurality ofstripe-shaped semiconductor layers stacked above the substrate andextending parallel to the substrate, each of the semiconductor layersbeing arranged between the insulating layers and having one end and another end parallel to the substrate, the one end being a source, and theother end being a drain; a gate electrode formed only on both, sidesurfaces of the semiconductor layers and extending perpendicular to thesubstrate; and a plurality of cell transistors formed at intersectionsof the gate electrode and the semiconductor layers and being driven bythe gate electrode in common, and wherein impurities are doped in eachof the semiconductor layers constituting the cell transistors uniformly,and the cell transistors are transistors of depletion type.
 2. Thememory according to claim 1, in which the other ends of the stackedsemiconductor layers form a stairs-shape, which further comprises aplurality of layer selecting transistors formed respectively on theother ends of the semiconductor layers of the stairs-shape and selectingeach of the semiconductor layers.
 3. The memory according to claim 1,further comprising a plurality of layer selecting transistors selectingeach of the semiconductor layers, each of the layer selectingtransistors being a tri-gate type in which a gate electrode covers a topsurface and the both side surfaces of the semiconductor layer.
 4. Thememory according to claim 1, wherein the drains of the semiconductorlayers are connected to an interconnection in common.
 5. The memoryaccording to claim 1, wherein the sources of the semiconductor layersare connected to an interconnection in common.
 6. The memory accordingto claim 1, wherein a number of the semiconductor layers is four ormore.
 7. The memory according to claim 1, wherein the cell transistorsare made of single-crystal silicon.
 8. The memory according to claim 1,wherein the semiconductor layers are formed in a recess dug down from atop surface of the substrate, and a top most surface of thesemiconductor layers is below the top surface of the substrate.
 9. Thememory according to claim 1, wherein a plurality of gate electrodesextending perpendicular to the substrate are formed only on sidesurfaces of the semiconductor layers and a plurality of cell transistorsformed at intersections of the plurality of gate electrodes and one ofthe semiconductor layers form a NAND chain of a NAND flash memory. 10.The memory according to claim 1, wherein the semiconductor layers areequal in thickness.
 11. The memory according to claim 1, wherein aplurality of stacked structures, each including the plurality ofstripe-shaped semiconductor layers and extending in parallel with eachother, are provided on the substrate, wherein a plurality of gateelectrodes extending perpendicular to the substrate are filled betweenthe plurality of stacked structures respectively, and the plurality ofgate electrodes are connected with each other at a top most surface ofthe semiconductor layers.